This invention relates to a method of forming patterns in the manufacture of microelectronic devices such as LSI devices and bubble memory devices, and more particularly to a pattern forming method which employs a dry etching technique for the transfer of resist patterns and utilizes a novel resist material.
In the manufacture of microelectronic devices such as semiconductor integrated circuit devices including LSI devices and bubble memory devices, optical lithography and electron-beam lithography are prevailing techniques to form fine patterns. With the recent tendency to make the patterns in such devices ever finer, dry etching processes such as gas plasma etching, reactive sputter etching and ion milling have been employed in place of the conventional wet etching processes for transferring resist patterns obtained by an exposure and development process to the substrates with high accuracy. Accordingly there is a keen demand for improved resist materials that are sufficiently endurant to dry etching and high in sensitivity and resolving capability.
The thickness of a resist layer is an important factor in realization of high resolution patterns. However, in the industrial manufacturing processes it is not rare for the surface of the substrate to be etched to have steps, and in such cases it is required to form a considerably thick resist layer in order to accomplish complete coverage of the steps and to provide a flat surface. This is unfavorable for attaining high resolution. When using a negative type resist, it is very difficult to form a high resolution pattern in a thick resist layer because swelling of the resist during the development treatment becomes significantly detrimental to the precision of the pattern. Also when using a positive type resist, high resolution patterning of a thick resist layer is difficult due to adverse influences of backscattering from the substrate in the case of electron-beam lithography and reflection from the substrate in the case of optical lithography. Particularly over the stepped areas of the substrate, significant variations in the resist pattern linewidth are liable to occur despite constantness of exposure by reason of extraordinarily strong proximity effects.
To solve such difficulties, J. M. Moran et al have proposed a three-layer technique in Journal of Vacuum Science and Technology, Vol. 16, No. 6, pp. 1620-1624 (1979). Accordingly to this three-layer technique the first layer which covers the subsrate surface and provides a flat surface is a sufficiently thick layer of an organic material, and the intermediate layer is formed of an inorganic material that can not easily be etched by dry etching using oxygen, such as silicon dioxide or silicon nitride. The third or top layer is a thin resist layer. In the patterning process, first the resist layer is exposed to light, X-ray or electron-beam and developed to generate a high resolution pattern. Next, the intermediate layer is subjected to dry etching with the resist pattern as a mask, and then the pattern of the intermediate layer is transferred into the thick organic layer by reactive sputter etching using oxygen gas. By employing this three-layer technique a high resolution pattern can be generated in the resist layer firstly because it is possible to use a desirably thin resist layer for pattern generation and secondly because unfavorable influences of backscattering of electrons from the substrate or reflection of light waves from the substrate can be avoided. The high resolution pattern can accurately be transferred into the thick organic layer by sequential etching.
However, from an industrial point of view it is a disadvantage of the three-layer technique that the processing operations become complicated and time-consuming mainly because of the addition of the intermediate layer which is formed by vacuum deposition, sputtering or plasma CVD method.
If it is practical to use a resist material that is endurant to dry etching using oxygen, it becomes possible to etch a thick organic layer by directly using the resist layer for initial patterning as a mask, and hence the above described three-layer structure can be simplified to a two-layer structure. To our knowledge, however, such a convenient resist material is not available in the present state of the art. Polydimethylsiloxane is known as endurant to dry etching to such extent that the etch rate of this material by O.sub.2 plasma etching is nearly zero. However, polydimethylsiloxane is unsuitable for practical use as a patterning resist material because this material is liquid at room temperature even in the form of coating film so that the film is liable to suffer from adhesion of dust particles, damages on the surface and/or inconvenience for handling attributed to its fluidity.